U.S. officials say the State Department has suffered a worldwide email outage affecting its entire unclassified system.

The officials said the outage began early Friday morning and that technicians were working to restore email as soon as possible. The officials said the outage was not caused by "any external action or interference."

The department was forced to shut down its unclassified email systems in 2014 in what officials said at the time was routine maintenance. But it later emerged that the system had been compromised by hackers believed to be affiliated with Russia and that the maintenance explanation was a cover story.